Continuing advancements in the electronics industry have imposed ever more rigorous purity requirements on cleanrooms where sensitive components are manufactured. Several years ago, class 100 cleanrooms (averaging no more than 100 particles of 0.5 microns diameter in one cubic foot of controlled air space) were acceptable, while requirements today often exceed class 11 based on 0.1 micron diameter particles. See, for example, prior art patents disclosing cleanroom structures include U.S. Pat. Nos. 3,158,457; 3,638,404; 4,667,579 and 4,693,173. Cleanroom ceilings, walls, and floors must therefore be constructed in such a manner as to minimize convection and eddy currents, dead air spots, and other areas which tend to collect dust and other particulate matter and/or disturb the uniform air flow in the cleanroom. Because of the moving air within the cleanroom, both convection currents and dead spots tend to form small, swirling pockets of air near the ceiling, referred to herein generally as vortices. These pockets capture particulate material and accumulate this contaminant, leading to breach of the class requirements for the cleanroom.
Generally, cleanrooms are adapted for generating uniform flow of filtered air from the ceiling to and through the floor. The air flow originates from blowers mounted above the ceiling on a support structure. The air from the blowers is forced through HEPA air filters forming the ceiling of the cleanroom and travels downwardly from the ceiling through the cleanroom, exiting through the floor. The ceiling filters are generally mounted on a grid of ceiling support beams or cross members, the bottom surfaces of which are in close proximity with the bottom surfaces of the filters.
Although the diffusion screen assists in developing laminar flow of the air exiting the filters, the desired uniform flow pattern is interrupted immediately below the ceiling surface by vortex regions beneath the cross members. These vortex regions form because of low pressure arising below the cross members in the absence of air flow, causing a dead space where particulate material can accumulate. The actual size and geometry of the vortex will vary, depending upon the width of the cross member and the velocity of air flow emanating from the adjacent filters.
The uniform flow pattern can also be disturbed by light fixtures and other attachments which are suspended from the cross members. For example, the high intensity lighting systems used in cleanrooms generally comprise extended linear arrays of fluorescent light tubes traversing the width and/or length of the cleanroom ceiling. The bottom surfaces of the support beams generally are used for the attachment of these light fixtures and are also used to attach mounting apparatuses for supporting modular walls and similar hardware. These attachments extend into the cleanroom from the ceiling plane formed by the ceiling filters and beams, creating convection currents and collection points for dust and other particulate matter which impair the purity of the cleanroom.
Efforts to place these light fixtures within the cross members have been frustrated by the need for minimizing the vortex by reducing the width of the cross member. Placement of the light fixture within the cross member would necessarily increase this width in order to provide adequate volume to fully contain the fixture. Accordingly, general practice continues to apply a tear drop configuration of lights which suspends the fixture below the cross member.
Nevertheless, the increasingly stringent requirements for minimal contamination within the cleanroom will likely necessitate modification of cleanroom ceiling structure to a flush mounted system. Brod McClung-Pace Co has introduced a flush ceiling system illustrated in FIG. 2 which depicts a widened cross member 10 having an enlarged channel 11 for receiving a light fixture 12. A gel track 13 supports HEPA filters 14 in a position located above the channel 11. Pace has attached a screen member 15 below the filter 14 in a manner which is represented to have reduced the vortex region 16 under the cross member to within 2 inches of the flush surface 17. Normally, a vortex will extend 3 to 4 times the grid width. The actual depth of the vortex associated with the Pace design is suggested to be only one-half the distance between adjacent filters. Accordingly, a separation distance between filters of 4 inches would result in a vortex region of two inches in depth. Although a two inch vortex may represent an improvement over the prior art, it still poses a formidable limitation to obtaining a desirable level of air purity for future cleanroom systems. In addition the Pace structure creates a new problem of air turbulence which is unresolved for the first seven to eight feet below the ceiling. This arises from the large openings around the periphery of their screen. These appear to generate enough turbulence to disturb laminar flow along this substantial length.
An additional area of concern with the Pace configuration is the placement of the gel track 13 above the cross member 10. This construction permits migration of particulate matter within lateral spaces 18. Not only does this present the possibility of contamination leakage to the cleanroom, but it operates to complicate actual detection of the leak location. Indeed, particles may travel several meters within the interconnecting channels 18 before escaping to the cleanroom interior. Although detection of this point of escape may be a simple procedure, the actual internal source of the leak may be very difficult to isolate.
On the other hand, placement of the respective gel tracks on opposite sides of the cross member would widen the separation space between filters as much as one inch. This would tend to lengthen the vortex region under traditional ceiling structure another 3 to 4 inches below the cross member. Therefore, the industry is caught in a balancing act of (1) placement of gel tracks above the cross member in order to narrow the separation distance between filters and (2) placement of the gel tracks at the base of the cross members to minimize contamination from migrating particles flowing within the ceiling structure, such as within open spaces 18. Neither choice offers the desired minimization of migration of contaminant particles. In one instance, migration occurs within passages of the ceiling support system. In the other case, the migration extends along a vortex located at the lower surface of the cross member or light fixture within the cleanroom.
As a further point of concern, no suitable arrangement of cleanroom ceiling fixture attachments has yet been developed which maximizes uniformity of noncontaminated air flow while at the same time offering compatibility with conventional cleanroom ceiling structure such as conventional HEPA filters with a lower mounting flange or knife edge positioned at the base of the filter. Note that the Pace "under slung" structure requires use of a special filter 14 whose mounting flange 15 is positioned at an upper portion 19 of the filter. Such compatibility with conventional low mounting flange or knife edge structure is not only important from a viewpoint of economy in construction, but the conventional filter with lower mounting flange offers a known advantage of better sealing which is known and trusted within the industry. Accordingly, the use of conventional HEPA filters avoids the formidable challenge of having to re-educate the market as to the acceptability of new filter structure, compared to that which is known and accepted.
Neither has such a system been developed for general use with flush lighting systems in ceilings of non-cleanroom environments, e.g., Lonseth, U.S. Pat. No. 4,175,281, Lipscomb, U.S. Pat. No. 3,173,616. To date, applicant is aware of no suitable fixture arrangement which has been developed which satisfies the specific requirements of a cleanroom, including minimizing the negative impact of vortex formation below the cross members without development of strong turbulence.